To what degree light emission can occur in a light emitting layer portion, which comprises an active layer and cladding layers, is an important index for performances of light emitting devices such as light emitting diode and semiconductor laser. An exemplary configuration popularly adopted is double heterostructure, in which the cladding layers and so forth, which function as confining and injecting carriers, are designed to be transparent to the light emission from the active layer (designed to have a band gap energy wider than a photon energy corresponded to the emission wavelength), and so as to sandwich the active layer. Increase in difference in the band gap energy between the active layer and cladding layers can improve internal quantum efficiency, and this consequently raises emission efficiency of the light emitting device.
On the other hand, improvement in external quantum energy (extraction efficiency of light out into the external of the light emitting device: simply referred to as light extraction efficiency, hereinafter) is also important, in addition to the improvement in the internal quantum efficiency. In consideration of the light extraction efficiency towards the external, difference in refractive index between different materials is an important issue. Assuming now that refractive index of the active layer as n1 and refractive index of the cladding layer as n2, a critical angle of total reflection θc of incident light from the active layer into the cladding layer is expressed asθc=Sin−1(n2/n1)  (i)The larger the difference in the refractive index between the different materials grows, the smaller the critical angle of total reflection on the interface between the different materials will be, therefore, in the above exemplary case, as the difference between refractive index n1 of the active layer and refractive index n2 of the cladding layer grows larger, the light emitted from the active layer will be more likely to cause total reflection on the interface with the cladding layer, and this consequently lowers the light extraction efficiency. The same will apply also to propagation of light from the cladding layer out into the external atmosphere.
The lowering in the light emission efficiency due to total reflection will be explained referring to FIGS. 7A and 7B. Assuming now that critical angle of total reflection at the interface between a crystal body and an external atmosphere (air) as θc, light having an angle of incidence smaller than θc will be extracted after transmitting through the interface out into the external (air) as shown in FIG. 7A. On the other hand, light having an angle of incidence larger than θc will totally be reflected on the interface as shown in FIG. 7B, and therefore will not be extracted outside the light emitting device.
To suppress the lowering in the light extraction efficiency due to total reflection, efforts have been made on reducing light energy to be totally reflected through fine processing of the device surface so as to increase the surface area, or on improving the light extraction efficiency through increasing light energy possibly extracted from the lateral sides by thickening layers other than the active layer, typically by thickening the cladding layers, for example. These methods are, however, disadvantageous in that they inevitably increase the number of process lines or material consumption, to thereby raise the production cost.
An object of this invention is therefore to provide a light emitting device having a desirable light extraction efficiency, and a method of fabricating the same.